Animals are thriving with microbial life, often establishing intimate symbiotic relationships with their tiny companions. When these persist over evolutionary time, they bring about deep changes to both the hosts and their symbionts. Since his early studies at the National Autonomous University of Mexico and subsequent PhD at the University of Valencia (Spain), Alejandro has been exploring the impact of symbiotic interactions on the biology and evolution of hosts and symbionts at the genomic, metabolic, and morphological levels. His research deals with animal-microbe symbioses, with a focus on two animal systems with nutrient-restricted diets: aphids and blood-feeding leeches. He approaches the study of these organisms using genomic, microscopic, and evolutionary methods.